Материал: ADM223

LOAD CURRENT – mA

Figure 25. Charge Pump V+, V– vs. Current

VCC – V

Figure 27. Transmitter Output Voltage vs. VCC

15

10

Tx O/P HI

5

0

–5

Tx O/P LO

–10

–15

IMPEDANCE –

350

300

V+ IMP

250

200

150

100

V– IMP

50

0

VCC – V

Figure 30. Charge Pump, V+, V– Exiting Shutdown

Figure 31. Transmitter Output Loaded Slew Rate

GENERAL INFORMATION

The ADM223/ADM230L–ADM241L family of RS-232 drivers/ receivers are designed to solve interface problems by meeting the EIA-232-E specifications while using a single digital 5 V supply. The EIA-232-E standard requires transmitters which will deliver ± 5 V minimum on the transmission channel and receivers which can accept signal levels down to ± 3 V. The ADM223/ADM230L–ADM241L meet these requirements by integrating step up voltage converters and level shifting transmitters and receivers onto the same chip. CMOS technology is used to keep the power dissipation to an absolute minimum. A comprehensive range of transmitter/receiver combinations is available to cover most communications needs.

The ADM223, ADM230L, ADM235L, ADM236L and ADM241L are particularly useful in battery powered systems as they feature a low power shutdown mode which reduces power dissipation to less than 5 W.

The ADM233L and ADM235L are designed for applications where space saving is important as the charge pump capacitors are molded into the package.

The ADM231L and ADM239L include only a negative charge pump converter and are intended for applications where a positive 12 V is available.

To facilitate sharing a common line or for connection to a microprocessor data bus the ADM235L, ADM236L, ADM239L and ADM241L feature an enable (EN, EN) function. When disabled, the receiver outputs are placed in a high impedance state.

CIRCUIT DESCRIPTION

The internal circuitry in the ADM230L–ADM241L consists of three main sections. These are:

(a)A charge pump voltage converter

(b)RS-232 to TTL/CMOS receivers

(c)TTL/CMOS to RS-232 transmitters

Charge Pump DC-DC Voltage Converter

The charge pump voltage converter consists of an oscillator and a switching matrix. The converter generates a ± 10 V supply from the input 5 V level. This is done in two stages using a switched capacitor technique as illustrated in Figures 33 and 34. First, the 5 V input supply is doubled to 10 V using capacitor C1 as the charge storage element. The 10 V level is then inverted to generate –10 V using C2 as the storage element.

VCC V+ = 2VCC

GND VCC

Figure 33. Charge-Pump Voltage Doubler

Figure 34. Charge-Pump Voltage Inverter

Capacitors C3 and C4 are used to reduce the output ripple. Their values are not critical and can be reduced if higher levels of ripple are acceptable. The charge pump capacitors C1 and C2 may also be reduced at the expense of higher output impedance on the V+ and V– supplies.

The V+ and V– supplies may also be used to power external circuitry if the current requirements are small.

Transmitter (Driver) Section

The drivers convert TTL/CMOS input levels into EIA-232-E output levels. With VCC = +5 V and driving a typical EIA-232-E load, the output voltage swing is ±9 V. Even under worst-case conditions the drivers are guaranteed to meet the ±5 V EIA-232-E minimum requirement.

The input threshold levels are both TTL and CMOS compatible with the switching threshold set at VCC/4. With a nominal VCC = 5 V the switching threshold is 1.25 V typical. Unused inputs may be left unconnected, as an internal 400 kΩ pull-up resistor pulls them high forcing the outputs into a low state.

As required by the EIA-232-E standard, the slew rate is limited to less than 30 V/µs without the need for an external slew limiting capacitor and the output impedance in the power-off state is greater than 300 Ω.

Receiver Section

The receivers are inverting level shifters which accept EIA-232-E input levels (±5 V to ±15 V) and translate them into 5 V TTL/ CMOS levels. The inputs have internal 5 kΩ pull-down resistors to ground and are also protected against overvoltages of up to

±30 V. The guaranteed switching thresholds are 0.8 V minimum and 2.4 V maximum which are well within the ±3 V EIA-232-E requirement. The low level threshold is deliberately positive as it ensures that an unconnected input will be interpreted as a low level.

The receivers have Schmitt trigger inputs with a hysteresis level of 0.5 V. This ensures error-free reception for both noisy inputs and for inputs with slow transition times.

Shutdown (SD)

The ADM223, ADM230L, ADM235L, ADM236L and ADM241L feature a control input that may be used to disable the part and reduce the power consumption to less than 5 µW. This is very useful in battery operated systems. During shutdown the charge pump is turned off, the transmitters are disabled

and all receivers except R4 and R5 on the ADM223 are put into a high-impedance disabled state. Receivers R4 and R5 on the ADM223 remain enabled during shutdown. This feature allows monitoring external activity such as ring indicator monitoring while the device is in a low power shutdown mode.

The shutdown control input is active high on all parts except the ADM223 where it is active low. Refer to Tables I and II.

Enable Input

The ADM235, ADM239, ADM241L and ADM223 feature an enable input used to enable or disable the receiver outputs. The enable input is active low on the ADM235L, ADM239L, ADM241L and active high on the ADM223. Refer to Tables I and II. When disabled, all receiver outputs are placed in a high impedance state. This function allows the outputs to be connected directly to a microprocessor data bus. It can also be used to allow receivers from different devices to share a common data line. The timing diagram for the enable function is shown in Figure 35.

*POLARITY OF EN IS REVERSED FOR ADM223.

Figure 35. Enable Timing

APPLICATION HINTS Driving Long Cables

In accordance with the EIA-232-E standard, long cables are permissible provided that the total load capacitance does not exceed 2500 pF. For longer cables which do exceed this, then it is possible to trade off baud rate vs. cable length. Large load capacitances cause a reduction in slew rate, and hence the maximum transmission baud rate is decreased. The ADM230L-ADM241L are designed so that the slew rate reduction with increasing load capacitance is minimized.

For the receivers, it is important that a high level of noise immunity be inbuilt so that slow rise and fall times do not cause multiple output transitions as the signal passes slowly through the transition region. The ADM230L-ADM241L have 0.5 V of hysteresis to guard against this. This ensures that, even in noisy environments, error-free reception can be achieved.

High Baud Rate Operation

The ADM230L-ADM241L feature high slew rates permitting data transmission at rates well in excess of the EIA-232-E specification. The drivers maintain ±5 V signal levels at data rates up to 100-kB/s under worst-case loading conditions.

BSC

BSC

14 8

NOTES

1.LEAD NO. 1 IDENTIFIED BY DOT OR NOTCH

2.PLASTIC LEADS WILL BE EITHER SOLDER DIPPED OR TIN PLATED IN ACCORDANCE WITH MIL-M-38510 REQUIREMENTS.

24-Lead Plastic DIP (N-24A)